In automobile engines, in order to prevent the escape of fuel vapor from a fuel tank into the atmosphere, the vapor is for example adsorbed by a canister of active carbon, and the adsorbed fuel is purged in the intake passage of the engine under predetermined running conditions. The fuel is removed from the canister and led into the intake passage by making use of the pressure difference between the atmosphere and the intake passage. For this purpose, the canister and intake passage are connected by a purge line, a purge valve which operates under predetermined running conditions being interposed in this line. Purge gas flowing into the intake passage from the purge line is led into the engine cylinder together with fuel injected by an injector provided in the intake passage, and burnt.
In many automobile engines which employ a three-way catalyst to process the engine exhaust gas, the air-fuel ratio (AFR) of the fuel mixture provided to the engine cylinder is feedback-controlled to stay in a certain region of a theoretically defined value. For example, a basic fuel amount is computed based on the intake volume of the engine, and this basic fuel injection amount is corrected based on the actual AFR detected by an oxygen sensor provided in an exhaust passage of the engine.
When the aforesaid purge gas is introduced, the fuel amount supplied to the engine cylinder increases by an amount corresponding to the purged fuel, and naturally the AFR becomes richer. In an engine with the above AFR feedback control system, the injection amount from the injector is corrected so that it is decreased.
If for example the detected AFR changes from lean to rich with reference to the theoretical value, an updating amount P is subtracted in one step from the feedback correction coefficient .alpha., and am updating amount I are then subtracted integrally until the AFR next changes back to lean. This is so-called "PI control". The correction coefficient .alpha. cam therefore be varied only in fixed proportions, and if the AFR is varying rapidly, a certain response time is required until the AFR can be made to converge to a target value.
If now the accelerator pedal is depressed so that the vehicle accelerates while fuel is being purged from the canister, the intake amount immediately increases, but the flowrate of purge gas does not vary so much. The fuel supplied to the engine is therefore insufficient due to the fact that the purge gas amount has decreased relative to the intake amount, so this deficiency is compensated by feedback control which increases the fuel amount injected by the injector.
Just before acceleration, however, the feedback control correction coefficient .alpha. had shifted from a center value of 1.0 to lean (e.g. 0.8) so as to make the AFR converge to the target value. A relatively long time is therefore required for the coefficient to change back to rich which is necessary to increase the fuel amount so that the vehicle can accelerate, and during this time the engine does not respond properly.
To deal with this problem, in Tokkai Hei 2-19631 published by the Japanese Patent Office, a method is proposed whereby the basic fuel amount is first decreased by a predetermined value while purge gas is being led into the cylinder, the AFR is modified to the target value (theoretical AFR) by feedback control, and the correction coefficient .alpha. is maintained close to the center value of 1.0 even during purge. For this purpose, the AFR correction coefficient .alpha. before starting purge is compared with the AFR correction coefficient .alpha. after purge which has fallen to a stable level below the predetermined value. By first subtracting a fuel correction amount corresponding to this difference from the basic fuel amount, the correction coefficient .alpha. during purge is thereby forcibly maintained in the region of 1.0 which is the center value. When the accelerator pedal is depressed, therefore, the feedback control correction coefficient .alpha. shifts to rich from the region of 1.0. Compared to the case when it shifts to rich from lean, the fuel amount increases more rapidly, hence the engine acceleration response is improved.
However, after starting purge, the basic fuel amount is only corrected after the correction coefficient .alpha. has almost stabilized to a constant value, and until this occurs, the only correction applied is that of feedback control. As mentioned hereintofore, the feedback correction coefficient .alpha. can be varied only in fixed proportions. Therefore, it cannot immediately respond to rapid fluctuations of AFR due to purge, and AFR errors tend to be large for a certain period after starting purge. Further, if the accelerator pedal is depressed during this period, the engine response worsens as stated heretofore.
Learning control is used to compensate for poor feedback control response. In learning control, the standing error of the AFR due to temporal variations of an air flow meter or the injector, is first corrected by learning. For example, if the injector becomes clogged due to long periods of use, the fuel amount is less for the same injection pulse width signal, so the AFR shifts to lean. If this AFR shift is memorized, a large pulse width is supplied to the injector when the vehicle is driven again based on the previously learned value, so there is no apparent change of injector flow characteristics from the time before clogging occurred. In other words, it is no longer necessary to apply feedback correction for decreases in fuel amount every time the injector clogs.
This learning control is referred to as learning control of the basic fuel amount, and it corrects the standard error of the AFR. However, if the AFR error due to purge is input by learning, there will be a large difference in the learning value depending on whether purge is or is not carried out. In this case, the learning process actually gives rise to more error, and AFR control is especially disturbed around the time when purge starts. In conventional learning control systems, therefore, learning was often prohibited during purge and was not useful for AFR control at that time.